Chapter 8: Heredity

Important questions — the ones the board asked most (20)

1. Q1Case-based4 marksAsked: 2023

   38. In order to trace the inheritance of traits Mendel crossed pea plants having one contrasting character or a pair of contrasting characters. When he crossed pea plants having round and yellow seeds with pea plants having wrinkled and green seeds, he observed that no plants with wrinkled and green seeds were obtained in the F1 generation. When the F1 generation pea plants were cross-bred by self-pollination, the F2 generation had seeds with different combinations of shape and colour also. (a) Write any two pairs of contrasting characteristics of pea plant used by Mendel other than those mentioned above. (1) (b) Differentiate between dominant and recessive traits. (1) (c) State the ratio of the combinations observed in the seeds of F2 generation (in the above case). What do you interpret from this result? (2) OR (c) Given below is a cross between a pure violet flowered pea plant (V) and a pure white flowered pea plant (v). Diagrammatically explain what type of progeny is obtained in F1 generation and F2 generation : Pure violet flowered plant x Pure white flowered plant (V V) (v v) (2)

   

   Medha Ma’am

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   Model answer · Mendel's laws, dominance and independent assortment

   1. 1.(a) Tall/dwarf plant and violet/white flower (or axial/terminal flower, green/yellow pod).1 mark
   2. 2.(b) A dominant trait is expressed in the F1 generation; a recessive trait stays hidden in F1 and is expressed only when both copies are recessive (in F2).1 mark
   3. 3.(c) In F2: 9 round-yellow : 3 round-green : 3 wrinkled-yellow : 1 wrinkled-green. OR (c) VV × vv: gametes V and v; all F1 plants are Vv, with violet flowers.1 mark
   4. 4.This shows that seed shape and seed colour are inherited independently, so new combinations (round-green, wrinkled-yellow) appear. OR Selfing F1 (Vv × Vv) gives VV, Vv, Vv, vv (1 : 2 : 1), so violet : white = 3 : 1.1 mark

   Marks split from a structure derived from past papers (checked twice); not CBSE’s official scheme

2. Q2Case-based4 marksAsked: 2024

   Mendel worked out the rules of heredity by working on garden pea using a number of visible contrasting characters. He conducted several experiments by making a cross with one or two pairs of contrasting characters of pea plant. On the basis of his observations he gave some interpretations which helped to study the mechanism of inheritance. (i) When Mendel crossed pea plants with pure tall and pure short characteristics to produce F1 progeny, which two observations were made by him in F1 plants? (ii) Write one difference between dominant and recessive trait. (iii) (A) In a cross with two pairs of contrasting characters RRYY X rryy (Round Yellow) (Wrinkled Green) Mendel observed 4 types of combinations in F2 generation. By which method did he obtain F2 generation ? Write the ratio of the parental combinations obtained and what conclusions were drawn from this experiment. OR (iii) (B) Justify the statement : "It is possible that a trait is inherited but may not be expressed."

   

   Medha Ma’am

   AskMedha’s AI teacher

   Hear it from Medha Ma’am · 150 s

   Model answer · Mendel's monohybrid and dihybrid crosses

   1. 1.(i) When a pure tall pea plant was crossed with a pure short pea plant, all plants in the F1 progeny were tall. No short plants were seen.0.5 marks
   2. 2.There were no halfway characteristics in the F1 progeny, so there were no 'medium-height' plants. Only one of the parental traits, tallness, was seen, not some mixture of the two.0.5 marks
   3. 3.(ii) Dominant trait: a single copy (for example 'T') is enough for the trait to be expressed; Recessive trait: it is expressed only when both copies are the same (for example 'tt').1 mark
   4. 4.(iii) (A) Mendel got the F2 generation by self-pollination of the F1 plants (Rr Yy, all round and yellow).0.5 marks
   5. 5.In the F2 progeny, round yellow, round green, wrinkled yellow and wrinkled green seeds appeared in the ratio 9 : 3 : 3 : 1. So the parental combinations, Round Yellow : Wrinkled Green, were in the ratio 9 : 1.0.5 marks
   6. 6.Conclusion: new combinations (round green and wrinkled yellow) also appeared in F2. This shows that the seed shape trait and the seed colour trait are independently inherited. In other words, traits are inherited independently, and this gives new combinations of traits in the offspring.1 mark

   • For (i), write both observations: "all plants in the F1 progeny were tall" and "no 'medium-height' plants".
   • For (iii)(A), name the method as "self-pollination of the F1 plants" and give the parental ratio as Round Yellow : Wrinkled Green = 9 : 1.
   • End (iii)(A) with the conclusion "traits are inherited independently".

   Marks split as in the board’s published marking scheme

3. Q3Case-based4 marksAsked: 2023

   The most obvious outcome of the reproductive process is the generation of individuals of similar design, but in sexual reproduction they may not be exactly alike. The resemblances as well as differences are marked. The rules of heredity determine the process by which traits and characteristics are reliably inherited. Many experiments have been done to study the rules of inheritance. (i) Why an offspring of human being is not a true copy of his parents in sexual reproduction ? (ii) While performing experiments on inheritance in plants, what is the difference between F1 and F2 generation ? (iii) (A) Why do we say that variations are useful for the survival of a species over time ? OR (iii) (B) Study Mendel's cross between two plants with a pair of contrasting characters. RRYY (Round Yellow) x rryy (Wrinkled Green) He observed 4 types of combinations in F2 generation. Which of these were new combinations ? Why do new features which are not present in the parents, appear in F2 generation ?

4. Q4Case-based4 marksAsked: 2025

   38. In human beings, there are 23 pairs of chromosomes. Out of these 23 pairs of chromosomes (i.e. 46 chromosomes), 22 pairs of chromosomes are called autosomes and one pair of chromosomes. i.e. two chromosomes are called sex chromosomes. The sex chromosomes are of two types - 'X' chromosomes and 'Y' chromosomes. The sex of a child (i.e. progeny), is decided at the time of fertilisation. In other words, at the time of zygote formation the sex chromosomes inherited from the parents of a child decide whether the new born will be a boy or a girl. (a) What are chromosomes ? (b) Why is the pair of sex chromosomes in human males called mismatched pair ? (c) (A) Show with the help of a flow chart that the statistical probability of getting a boy or a girl is 50 : 50. OR (c) (B) Mention two examples of animals where sex is not determined genetically like human beings. Describe in brief the method of sex determination in these animals.

   How the model answer begins

   1. (a) Chromosomes are separate, independent pieces of DNA that together make up a gene set. They carry genes, which control the traits of an organism. Through DNA, chromosomes carry the information for passing features…

   The full answer from Medha Ma’am →
5. Q5Case-based4 marksAsked: 2023

   In some families, either rural or urban, females are tortured for giving birth to a female child. They do not seem to understand the scientific reason behind the birth of a boy or a girl. In fact the mother is not responsible for the sex of the child and it has been genetically proved that the sex of a newborn is determined by what the child inherits from the father. (a) State the basis on which the sex of a newborn baby is determined in humans. (b) Why is the pair of sex chromosomes called a mismatched pair in males ? (c) How is the original number of chromosomes present in the parents restored in the progeny ? OR (c) Explain by giving two examples of the organisms in which the sex is not genetically determined.

   How the model answer begins

   1. (a) Every child gets an X chromosome from the mother, so the sex depends on the chromosome from the father: an X gives a girl (XX) and a Y gives a boy (XY).

   The full answer from Medha Ma’am →
6. Q6Short answer3 marksAsked: 2026

   (a) A couple are parents to 4 daughters in a sequence, and do not have any son. Does this indicate that the husband does not produced Y-chromosome bearing sperms ? Explain. (b) What are the chances of this couple bearing yet another daughter ? Show with the help of a cross.

   How the model answer begins

   1. No, this is not an indication that the husband is not producing sperms containing Y chromosome. Men have a mismatched pair of sex chromosomes, X and Y, so the husband produces both X-chromosome carrying sperms and…

   The full answer from Medha Ma’am →
7. Q7Short answer3 marksAsked: 2026

   Show the cross and observations made by Mendel when he crossed pure tall pea plant with a pure short pea plant to get F1 progeny.

   How the model answer begins

   1. Mendel crossed a pure tall pea plant (TT) with a pure short pea plant (tt). Parents: TT (tall) × tt (short) Germ cells: T and t F1: Tt

   The full answer from Medha Ma’am →
8. Q8Short answer3 marksAsked: 2026

   A blue colour flower (BB) bearing plant is crossed with a plant with white coloured flowers (bb). (a) State the colour of the flowers you would expect in their F_1 progeny. (b) What must be the percentage of white flower plants in the F_2 generation, if the F_1 plants are self-pollinated ? (c) State the expected ratio of genotypes BB and bb in F_2 progeny.

   How the model answer begins

   1. (a) All the F1 progeny will have blue coloured flowers. The blue parent (BB) gives gene B and the white parent (bb) gives gene b, so every F1 plant is Bb. Blue is the dominant trait, so only blue is expressed. White is…

   The full answer from Medha Ma’am →
9. Q9Short answer3 marksAsked: 2026

   Study the given table and answer the following questions : | Parents | F1 | F2 | | --- | --- | --- | | A pea plant with red flowers was crossed with a pea plant with white flowers. | 30 seeds were sown and all bore plants with red flowers. | 44 seeds were sown, 33 plants bore red flowers and 11 plants bore white flowers. | (a) Depict the information given in the table in the form of a cross. (b) Deduce the F2 phenotypic ratio/F2 ratio showing external look as well as genotypic /genetic makeup, ratio.

   How the model answer begins

   1. (a) Parents: RR (red flowers) × rr (white flowers). R is the dominant gene for red flowers and r is the recessive gene for white flowers.

   The full answer from Medha Ma’am →
10. Q10Short answer3 marksAsked: 2025

    The gene combination of purple flowered pea plants is denoted as (WW) and that of white flowered pea plants as (ww), when these two plants are crossed F1 generation is obtained. (a) List two observations made by Mendel in F1 generation plants. (b) Give the (i) percentage of white flowered plants and (ii) ratio of the gene combinations WW, Ww and ww in F2 generation. (c) Write one difference between dominant and recessive trait.

    How the model answer begins

    1. (a) Two observations Mendel made in the F1 generation (Ww): 1. All plants were purple flowered, and no white flowered plants were seen. 2. There were no halfway characteristics, so no mixed coloured flowers appeared.…

    The full answer from Medha Ma’am →
11. Q11Short answer3 marksAsked: 2025

    (a) "In human beings the genes inherited from the parents decide whether the newborn individual is male or female." Explain this statement with the help of a flow diagram. (b) "Some animals rely on environmental cues for sex determination." Justify this statement giving an example.

    How the model answer begins

    1. (a) The mother is XX, so all her eggs carry an X chromosome; the father is XY, so half his sperms carry X and half carry Y.

    The full answer from Medha Ma’am →
12. Q12Short answer3 marksAsked: 2024

    30. List two differences between dominant traits and recessive traits. What percentage of pea plants in the F2 generation were with yellow seeds in Mendel's cross between the pea plants having yellow (YY) and green coloured (yy) seeds ?

    How the model answer begins

    1. Dominant traits: (i) When both the dominant and the recessive traits are inherited, the dominant trait gets expressed. (ii) A single copy of the dominant trait is enough to get it expressed. For example, both TT and Tt…

    The full answer from Medha Ma’am →
13. Q13Short answer3 marksAsked: 2026

    (a) Giving two examples, justify the statement that the sex of the offspring in all animals is not always genetically determined. (b) Differentiate between the sex chromosomes of male and female human beings.

    How the model answer begins

    1. (a) The sex of the offspring is not always genetically determined because some species rely entirely on environmental cues. In a few reptiles, the temperature at which fertilised eggs are kept determines whether the…

    The full answer from Medha Ma’am →
14. Q14Short answer3 marksAsked: 2026

    Based on Mendel's experiment, make a cross up to F2 generation in which a pea plant with round and yellow seeds (RRYY) was crossed with a plant with wrinkled and green seeds (rryy). Write the number of seeds/plants with the following gene combinations (genotype) in F2 generation : (i) RRYY (ii) rryy

    How the model answer begins

    1. Parents: RRYY (round, yellow) × rryy (wrinkled, green) Gametes: RY × ry F1 generation: RrYy (round, yellow). All F1 plants have round, yellow seeds, so round and yellow are dominant traits. The F1 plants are used to…

    The full answer from Medha Ma’am →
15. Q15Short answer3 marksAsked: 2025

    In one of Mendalian experiments, when F1 generation pea plants with round yellow seeds were self-pollinated, pea seeds with the following combinations were obtained in F2 generation : [passage] Analyse the result and describe the mechanism of inheritance of traits which explains the above results.

    How the model answer begins

    1. When the F1 plants with round yellow seeds were self-pollinated, the F2 generation had four kinds of seeds: round yellow, round green, wrinkled yellow and wrinkled green. They were in the ratio 9 : 3 : 3 : 1 (315 : 108…

    The full answer from Medha Ma’am →
16. Q16Short answer3 marksAsked: 2025

    The lower part of the ear, called the earlobe, is completely attached to the side of the head in some people (Diagram 'X') and in some others this part is not completely attached, called free earlobe (Diagram 'Y'). Free and attached earlobes are two variants found in human population. The gene for free earlobe is dominant over the gene for attached earlobe. (a) A person with attached earlobe marries a woman with free earlobe. 50% of their children have free earlobes and the other 50% have attached earlobes. Explain the inheritance of this trait and write the gene-combination of the progeny. (b) Write the gene-combination of the father and mother in the above case.

    How the model answer begins

    1. (a) Free earlobe is the dominant trait: Ff. Attached earlobe is the recessive trait: ff. Each child gets one copy of the gene from the father and one from the mother. F (free) is expressed whenever it is present.…

    The full answer from Medha Ma’am →
17. Q17Short answer3 marksAsked: 2025

    30. A pure pea plant bearing terminal flowers was cross-pollinated with a pure plant having axillary flowers. In F1 generation, plants with axial flowers only were obtained. F1 generation plants are self-pollinated and F2 generation is obtained. (a) Work out the pattern of inheritance in this case. (b) What will be the ratio of plants obtained in F2 generation ?

    How the model answer begins

    1. (a) Let A stand for the gene for axial flowers and a for the gene for terminal flowers. Parental generation: pure axial-flowered plant (AA) × pure terminal-flowered plant (aa) Gametes: A and a F1 generation: all plants…

    The full answer from Medha Ma’am →
18. Q18Short answer3 marksAsked: 2025

    A pure pea plant having round (R), yellow (Y) seeds is crossed with another pure pea plant having wrinkled (r), green (y) seeds. Subsequently F1 progeny is self-pollinated to obtain F2 progeny. (a) What do the seeds of F1 generation look like ? (b) Give the possible combinations of traits in seeds of F2 generation. Also give their ratio. (c) State the reason of obtaining seeds of new combination of traits in F2 generation.

    How the model answer begins

    1. (a) All the seeds of the F1 generation are round and yellow (Rr Yy). Round and yellow are the dominant traits, so only they are seen in the F1 progeny. Wrinkled and green are not expressed.

    The full answer from Medha Ma’am →
19. Q19Short answer3 marksAsked: 2024

    Mendel crossed pure tall pea plants (TT) with pure short pea plants (tt) and obtained F1 progeny. When the plants of F1 progeny were self-pollinated, plants of F2 progeny were obtained. (a) What did the plants of F1 progeny look like ? Give their gene combination. (b) Why could the gene for shortness not be expressed in plants of F1 progeny ? (c) Write the ratio of the plants obtained in F2 progeny and state the conclusion that can be drawn from this experiment.

20. Q20Long answer2 marksAsked: 2026

    15. Mendel took garden pea plants with different characteristics, such as height to study the inheritance pattern of factors (genes). He crossed tall pea plant with short pea plant and obtained all the tall plants in the F1 generation. Answer the following questions : (a) Why only tall pea plants were observed in F1 progeny ? (b) By which method did Mendel obtain F2 progeny ? (c) (i) Write one difference between dominant and recessive trait. OR (c) (ii) Write two observations made by Mendel about F1 progeny.

    How the model answer begins

    1. (a) Only tall plants appeared in F1 because tallness is dominant over dwarfness: every F1 plant is Tt, and one copy of T is enough to make it tall.

    The full answer from Medha Ma’am →

10 more questions of this chapter, each with its model answer, in the AskMedha app.

Objective questions with the board’s key (10)

1. 1. CBSE 2026

   Choose the correct statements with reference to chromosomes : (i) carry hereditary information from parents to next generation. (ii) are thread-like structures located inside the nucleus of an animal cell. (iii) always exist in pairs in human gametes. (iv) are involved in the process of cell division.

   • (A) (i) and (ii)
   • (B) (iii) and (iv)
   • (C) (i), (ii) and (iv) ✓
   • (D) (ii), (iii) and (iv)

   Answer: (C)

2. 2. CBSE 2026

   Identify the traits that cannot be transmitted from parents to the off-springs : (i) Swimming technique (ii) Blood group (iii) Eye colour (iv) Dancing skills Options :

   • (A) (i) and (ii)
   • (B) (i) and (iv) ✓
   • (C) (iii) and (iv)
   • (D) (ii) and (iii)

   Answer: (B)

3. 3. CBSE 2026

   Two pea plants, one with round green seeds (RRyy) and another with wrinkled yellow (rrYY) seeds were crossed with each other which produced F1 progeny that have only round yellow (RrYy) seeds. When F1 plants are self-pollinated, the F2 progeny will have which new combination of characters, as compared to the parents : (i) Round, yellow (ii) Round, green (iii) Wrinkled, yellow (iv) Wrinkled, green Correct options are :

   • (A) (i) and (ii)
   • (B) (i) and (iv) ✓
   • (C) (ii) and (iv)
   • (D) (i) and (iii)

   Answer: (B)

4. 4. CBSE 2026

   Assertion (A) : Variations are not often seen in off-springs produced by asexual reproduction. Reason (R) : DNA molecule generated by replication is not similar to the original DNA.

   • (A) Both Assertion (A) and Reason (R) are true and Reason (R) is the correct explanation of the Assertion (A).
   • (B) Both Assertion (A) and Reason (R) are true, but Reason (R) is not the correct explanation of the Assertion (A).
   • (C) Assertion (A) is true, but Reason (R) is false. ✓
   • (D) Assertion (A) is false, but Reason (R) is true.

   Answer: (C)

5. 5. CBSE 2026

   Sex is determined by different factors in various species. However, in human beings, it is determined genetically. Which amongst the following option(s) is/are correct for human beings ? (i) Gamete carrying X chromosome from female parent. (ii) Gamete carrying X chromosome from male parent. (iii) Gamete carrying Y chromosome from male parent.

   • (A) (ii) and (iii) ✓
   • (B) (i) only
   • (C) (i) and (iii)
   • (D) (iii) only

   Answer: (A)

6. 6. CBSE 2026

   The genotype of green stemmed tomato plant is denoted by ‘GG’ and that of purple stemmed tomato plant as ‘gg’. Hybrid of a cross between these two were self-pollinated to obtain F2 progeny. What will be the ratio of GG, Gg and gg in this F2 progeny ?

   • (A) 2 : 1 : 1
   • (B) 3 : 1 : 0
   • (C) 1 : 1 : 2
   • (D) 1 : 2 : 1 ✓

   Answer: (D)

7. 7. CBSE 2026

   When a human egg is fertilized by a sperm having 'Y' chromosome, the zygote has the following combination of chromosomes :

   • (A) 44 + XX
   • (B) 22 + XX
   • (C) 44 + XY ✓
   • (D) 22 + XY

   Answer: (C)

8. 8. CBSE 2026

   Assertion (A) : Bacteria that can withstand heat have better chances of survival in a heat wave. Reason (R) : Accumulation of variations in a species increases the chances of its survival in changing environment.

   • (A) Both Assertion (A) and Reason (R) are true and Reason (R) is the correct explanation of the Assertion (A). ✓
   • (B) Both Assertion (A) and Reason (R) are true, but Reason (R) is not the correct explanation of the Assertion (A).
   • (C) Assertion (A) is true, but Reason (R) is false.
   • (D) Assertion (A) is false, but Reason (R) is true.

   Answer: (A)

9. 9. CBSE 2025

   When a pure-tall pea plant is crossed with a pure-dwarf pea plant, the percentage of tall pea plants in F1 and F2 generation pea plants will be respectively :

   • (A) 100% ; 25%
   • (B) 100% ; 50%
   • (C) 100% ; 75% ✓
   • (D) 100% ; 100%

   Answer: (C)

10. 10. CBSE 2025

    Assertion (A): In reptiles, the sex of the offspring is determined by the incubation temperature of the fertilized eggs (zygotes). Reason (R): In some organisms, sex determination is not genetic.

    • (A) Assertion (A) and Reason (R) are true and Reason (R) is the correct explanation of Assertion (A). ✓
    • (B) Assertion (A) and Reason (R) are true but Reason (R) is not the correct explanation of Assertion (A).
    • (C) Assertion (A) is true but Reason (R) is false.
    • (D) Assertion (A) is false but Reason (R) is true.

    Answer: (A)

Questions parents and students ask

How many marks does "Heredity" carry in CBSE Class 10 Science?

In the CBSE papers of 2023–2026, this chapter carried about 5 marks and about 2 questions a year; by marks it ranks 13 in Science.

Which are the most important (VVI) questions of "Heredity"?

The 20 questions above are from the board’s real papers, each with the years it was asked; the most-asked come first with their full model answers. All 30 written questions of this chapter, with answers, are in the AskMedha app.

Do the answers follow the board’s marking scheme?

Yes. CBSE publishes its marking schemes; every model answer here is written point by point to the marks the board’s own scheme gives, and checked by a second, independent model.